Add unpkg badge to readme

Unsure how best to "advertise" the dist download. Custom badge for now.

Link points to dist/ dir at latest published version.

Author: jasonkarns

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/testdouble/testdouble.js.svg?branch=master)](https://travis-ci.org/testdouble/testdouble.js)
 [![npmjs](https://img.shields.io/badge/npm-testdouble-red.svg)](https://www.npmjs.com/package/testdouble)
+[![unpkg](https://img.shields.io/badge/unpkg-download-blue.svg)](https://unpkg.com/testdouble/dist/)
 [![Test Coverage](https://codeclimate.com/github/testdouble/testdouble.js/badges/coverage.svg)](https://codeclimate.com/github/testdouble/testdouble.js/coverage)
 
 Welcome! Are you writing JavaScript tests and in the market for a mocking library to